Current compensation method and device for power system protection

ABSTRACT

The present invention relates to a method for current compensation of a protection system for protecting a zone in a power system, which zone comprises a number of transmission lines connected to power sources and a number of transmission lines connected to a number of loads where the power sources and the loads are arranged outside the zone and a number of current transformers (CT) arranged to the transmission lines, wherein the method comprises the steps of continuously measuring all the incoming currents (I in ) to the zone, continuously measuring all the outgoing currents (I out ) from the zone, continuously calculating the differential current (I d ) according to I d =I in −I out , continuously calculating q=I out /I in , continuously comparing q with set values and continuously comparing I out  with a set value Set1. The invention is characterised by setting I out =I in =I comp  at the instant when q changes and I out  exceeds the value set1, which is an indication of the occurrence of a fault external to the protection zone due to saturation of the CT of the faulted outgoing transmission line, and using the value I comp  in the protection system in order to prevent tripping of the protection system. The present invention also relates to a device and a computer program product for performing the method.

TECHNICAL FIELD

The present invention relates to a method for current compensation of aprotection system for protecting a zone in a power system, which zonecomprises a number of transmission lines connected to power sources anda number of transmission lines connected to a number of loads where thepower sources and the loads are arranged outside the zone and a numberof current transformers (CT) arranged to the transmission lines, whereinthe method comprises the steps of continuously measuring all theincoming currents (I_(in)) to the zone, continuously measuring all theoutgoing currents (I_(out)) from the zone, continuously calculating thedifferential current (Id) according to I_(d)=I_(in)−I_(out),continuously calculating q=I_(out)/I_(in), and continuously comparing qwith a set value S.

BACKGROUND OF THE INVENTION

During a number of years there has been a rapid development in powersystems and the capacity requirements of these in turn require highlyreliable relaying principles for protecting the system or components ofthe system in case of faults. These protection requirements apply tomany parts of the power system such as for example transformerdifferential protection, motor differential protection, generatordifferential protection and busbar protection.

In this kind of protection system, the incoming and outgoing currents ofa certain protection zone have been measured since these may be used todetect if a fault occurs within or outside the protection zone. In orderto measure these currents, so called current transformers, or CT, areused, one on each incoming or outgoing line. Further each line isprovided with a circuit breaker for breaking the line in case of afault.

Digital protection systems have been developed to monitor a powersystem. These protection systems not only requires fast operation speedfor heavy fault currents, but also need to be stable for external faultswhich are close to the protection zone. As there are a lot of differentcurrent transformers connected to the feed lines and there is noimpedance to limit the fault current within the zone, it might be a verysevere CT saturation condition in case of an external fault close to theCT. The very heavy CT saturation will produce an inaccurate currentvalue and thus a wrong picture for the type of differential protectionsystems used currently. As a consequence, the differential protectionmight misoperate in case of an external fault and thereby trip forprotection against a non-existent internal fault, especially for heavyCT saturation conditions.

A very difficult technical problem for this type of protection system iscalled simultaneous faults. This means that an internal fault occursfollowing an external fault and it is not possible to produce a tripsignal with current differential protection methods because there areenormous crossing currents while the internal fault is taking place. Theworst case occurs when the external fault current is equal to theinternal fault current. In this case both fault currents share thesource current and the differential current will have a large differencecompared with the restrained current.

The modern low impedance differential protection algorithm used can beexpressed as follows. If we suppose a passive connection point with Ntransmision lines, I_(d) represents the differential current and I_(r)represents the restrained current among those lines. $\begin{matrix}{I_{d} = {{\sum\limits_{i = 1}^{N}\;{Ii}}}} & (1) \\{I_{r} = {\sum\limits_{i = 1}^{N}\;{{Ii}}}} & (2)\end{matrix}$ I _(d) −k×I _(r) >D(3)

In case of internal fault we have I_(d)=I_(r) so that equation (3) canbe confirmed if we set the proper k value (k<1) and D value. Equation(3) is known as percentage differential protection since it introducesthe restrained current in order to make protection more stable forexternal faults.

In case of normal load or external faults, I_(d) should be zero so thatthe equation (3) is not satisfied. As a consequence there will not be atrip signal issued according to Kirchhoff's first law. In reality, I_(d)is still larger than zero for external fault cases during CT saturationperiod so that a misoperation will be produced during this time period.

The main technical problems for the algorithms used with digitaldifferential protection systems is misoperation due to external faultsclose to the feeder CT's especially in case of different CT cores. Inthis case, the saturation of CT in the faulted line will produceinaccurate current values similar to an internal fault in the measuringcircuits, that is, the differential current I_(d) will be the same asthe restrained current I_(r) during the CT saturation period when anexternal fault occurs.

In the case of busbar protection, a further drawback with someprotection systems is that CT saturation is compensated in each bay ofthe system. This means that there could be a plurality of measuringdevices for a large power system area, which is costly and ineffective.

BRIEF DESCRIPTION OF THE INVENTION

The object of the present invention is to provide a protection systemwhich can provide reliable protection also for very difficultconditions, such as external faults close to feeder CT's or simultaneousfault cases.

This object is achieved by the method according to claim 1, the deviceaccording to claim 8 and the computer program product according to claim10. Further aspects of the present invention are covered by theindependent claims.

The benefits of the present invention are several. It is based oncontinuously monitoring the ratio between the outgoing and incomingcurrent of a protection zone. In case of an external fault, because theCT saturates due to very high outgoing current, in which saturation mayaffect the protection system so that a tripping signal is issuedwrongly, the ratio provides an indication of the point when the CTsaturates. The present invention then puts the outgoing current equal tothe incoming current in order to compensate for the influence of theexternal fault on the protection system. The compensated value of theoutgoing current is then used to calculate new differential andrestrained currents, on which the tripping algorithm is based.

This provides a much more stable protection algorithm for externalfaults compared to the state of the art, thus greatly reducing the riskfor misoperation of the protection system.

The present invention also provides a method comprising an algorithm fordetecting an internal fault during an occurring external fault, which upto now has been virtually impossible to detect. The algorithm is basedon the facts

-   -   that the instant value of the outgoing current does not follow        the incoming current after zero crossing, which should be the        case when an external fault occurs, i e the incoming current and        the outgoing current should be practically the same,    -   the occurrence of a differential current. In external fault        cases there is no differential current before CT saturation, and        thus the presence of a differential current is an indication of        an internal fault. By continuously monitoring the values of        these currents, calculating the integrated values and comparing        them with previous values and/or set values, an internal fault        can be accurately detected during an external fault.

A further advantage with the present invention is the approach toinclude all incoming currents and outgoing currents of a protection zoneinstead of monitoring in each bay or for each equipment.

These and other benefits and aspects of the present invention willbecome apparent from the detailed description of the invention inconnection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following detailed description of the present invention,reference will be made to the accompanying drawings, of which

FIG. 1 shows an example of the behaviour of incoming and outgoingcurrents for an internal fault which occurs in a protection zone,

FIG. 2 shows an example of the behaviour of incoming and outgoingcurrents for an external fault which occurs outside the protection zone,

FIG. 3 shows an example of differential and restrained currents for theexternal fault,

FIG. 4 shows current compensation logic according to the presentinvention,

FIG. 5 shows an example of compensation results obtained with thepresent invention for external fault conditions,

FIG. 6 shows schematically a device for performing the method accordingto the invention, and

FIG. 7 shows schematically the principle of a protection zone accordingto the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to protection of power systems, and inparticular to areas of power systems having no sources or loads withinthose areas. These areas will hereafter be named protection zones PZ.Within these zones a number of feed lines connected to external sourcesare arranged as well as a number of feed lines connected to externalloads. External in this context means outside the protection zone. Theprotection zone does not contain any sources or loads and can be seen asa passive part of a power system. The protection zone could compriseeverything from one to a plurality of bays, busbars, equipment and thelike.

In FIG. 7 the principle of the protection zone PZ is shownschematically. The total current from all sources entering the zone isreferred to as I_(in) and the total current to all loads from PZ isreferred to as I_(out). The currents are conventionally measured bycurrent transformers CT. For a given PZ it is quite clear that allincoming currents have to be equal to the outgoing currents in normalload cases, when the PZ is defined as above, i e I_(in)=I_(out) orI_(out)/I_(in)=1. This should also be true if an external fault occurs.

If one phase is considered in a PZ and we suppose that N feed lines arepresent in a certain PZ, the incoming current I_(in) and outgoingcurrent I_(out) of the phase can be obtained by equations (4) and (5):$\begin{matrix}{I_{in} = {{\sum\limits_{i = 1}^{M}\;{Ii}}}} & (4) \\{I_{out} = {{\sum\limits_{i = {M + 1}}^{N}\;{Ii}}}} & (5)\end{matrix}$

Here, the index i from 1 to M corresponds to the incoming currents to PZand i from M+1 to N corresponds to the outgoing currents from theprotection zone.

The instantaneous values of the differential current I_(d) and therestrained current I_(r) can be expressed by I_(in) and I_(out) asI _(d) =I _(in) −I _(out)  (6)I _(r) =I _(in) +I _(out)  (7)

In order to have stable values of the incoming current I_(in) and theoutgoing current I_(out) for a certain protection zone, integratedvalues of these currents as well as I_(d) and I_(r) can be obtained bycontinuous integration over each fundamental frequency cycle T as$\begin{matrix}{I_{IN} = {\int_{t1}^{({{t1} + T})}{I_{in}\ {\mathbb{d}t}}}} & (8) \\{I_{OUT} = {\int_{t1}^{({{t1} + T})}{I_{out}\ {\mathbb{d}t}}}} & (9) \\{I_{D} = {\int_{t1}^{({{t1} + T})}{I_{d}\ {\mathbb{d}t}}}} & (10) \\{I_{R} = {\int_{t1}^{({{t1} + T})}{I_{r}\ {\mathbb{d}t}}}} & (11)\end{matrix}$

By using the integrated values from the equations (8) to (11), analgorithm can be formed, by which faults inside the protection zone aredetected very fast and by means of which a very fast tripping signal canbe generated, disconnecting the zone from the power system.

For most power systems, in case of serious faults, tripping must be donevery quickly because of the stability of the system but also in order toprevent serious damage. Preferably a tripping signal should be producedwithin 5 ms following internal faults.

This may be achieved with the present invention by using the rate ofchange of the integrated continuous values of I_(IN), I_(OUT) and I_(D).The fact is that all of these three integration values are one variablefunction in the time domain if a continuous integration is performed.This means that integration values will change depending on when theintegration is performed. If we suppose thatk ₁(t)=d(I _(D)(t))/dtk ₂(t)=d(I _(IN)(t))/dtk ₃(t)=d(I _(OUT)(t))/dt  (12)where k₁, k₂, k₃ are rate of change values. If a discrete time domainsystem is used, the rate of change values may be expressed ask ₁(i)=I _(D)(i)−I _(D)(i−1)k ₂(i)=I _(IN)(i)−I _(IN)(i−1)k ₃(i)=I _(OUT)(i)−I _(OUT)(i−1)  (13)

Here, index i corresponds to the sampling instant in the discrete timedomain and i−1 corresponds to the previous sampling time.

It has been shown that there exists differences for the factors k₁(i),k₂(i) and k₃(i) for different cases such as normal load, external faultsand internal faults. This is shown in table 1 below.

Normal load cases External fault cases Internal fault cases k₁(i) = 0k₁(i) increases after k₁(i) increases saturation of current transformerk₂(i) = 0 k₂(i) increases k₂(i) increases k₃(i) = 0 k₃(i) increasesbefore k₃(i) decreases saturation of current transformer

By continuously monitoring the rate of change values k₁, k₂ and k₃ alogic may be created for producing a fast tripping signal. The abovedescribed fast tripping system is described in detail in the SwedishPatent Application No SE0001436-5, which application hereby is includedin its entirety by reference.

External Faults

From a theoretical point of view, it is always true that the outgoingcurrent I_(OUT) equals the incoming current I_(IN) for normal loadconditions and for external faults. In reality, the outgoing currentwill not be equal to the incoming current once the corresponding CT'ssaturate. The worst cases of CT saturation occurs during external faultconditions where the fault current in the faulted location might causethe CT to saturate after 1 ms. If the CT saturates, the outgoing currentwill be equal to zero during CT saturation period and the differentialcurrent will be equal to the restrained current during CT saturationperiod. As a result the traditional percentage differential protectionalgorithm will misoperate.

The actual outgoing current and incoming current in internal andexternal fault cases are shown in FIGS. 1 and 2 respectively. In thefigures, the curve marked with □ represents the incoming current and thecurve marked with o represents the outgoing current. The differentialcurrent and restrained current are shown in FIG. 3. In that figure, thecurve marked with □ represents the restrained current and the curvemarked with o represents the differential current.

In order to make a very stable differential protection algorithm, acurrent compensation algorithm is proposed. It involves compensating theoutgoing current value based on the incoming current value duringexternal fault cases in order to always keep the differential currentclose to zero during external fault cases. As a consequence, a stablealgorithm can be formed even for very serious CT saturation cases.

In order to compensate the outgoing current with the incoming currentduring external fault cases, it has been found that the outgoing currentmust equal the incoming current both during normal load conditions andexternal fault conditions. If the CT saturates, the outgoing currentwill be equal to zero so that outgoing current will not be equal to theincoming current. From FIG. 2 it can be derived that there is a shorttime period At that the outgoing current equals the incoming currentbefore the CT saturates for external fault cases.

A compensation algorithm can be formed according to the outgoing currentand incoming current wave forms shown in FIG. 2. The ratioI_(out)/I_(in) and the changes of outgoing current and incoming currentare used to detect the CT saturation in external fault cases. Once thereis a CT saturation, I_(out)/I_(in) will drop sharply and I_(out) willdecrease so that the current compensation must be made from this timeinstant. In order to avoid the miscompensation during normal loadcondition and internal fault conditions, a pickup value is used to checkif the outgoing current is larger than a preset value.

The total current compensation logic is shown in FIG. 4. Here the ratioq=I_(out)/I_(in) is compared to a set value of 0.6. For ideal conditionsthe value should be 1, but due to measuring and other errors andinfluences, it is set somewhat lower. i denotes the present time and i−1and i−2 the pervious sampling times. Thus two previous sampling valuesof q are compared against the set value of 0.6. If the sampled valuesq(i−1) and q(i−2) are both larger than 0.6, and present time q(i) isless than 0.8, a signal is sent to an AND gate.

Further the value of the outgoing current I_(out)(i−1) of the previoussampling is compared to a set value set1. If I_(out)(i−1) is larger thanset1, where set1 is based on pre-fault outgoing current values, a signalis sent to the AND gate.

The value of I_(in) of the present sampling, I_(in)(i) is also comparedthe previous sampling value I_(in)(i−1), and if the present value isgreater than the previous value, a signal will be sent to the AND gate.

A further logic unit compares the previous value of I_(out) with a yetprevious value of I_(out), I_(out)(i−1)>I_(out)(i−2), and comparesI_(out)(i−1)×0.4 with the previous value of I_(d),I_(out)(i−1)×0.4>I_(d)(i−1). If those requirements are fulfilled, asignal will be issued to the AND gate.

If all above conditions are met, i eq(i−2)>0.6, q(i−1)>0.6, q(i)<0.8  (12)ANDI _(out)(i−1)>set1  (13)ANDI _(in)(i)>I_(in)(i−1)  (14)ANDI _(out)(i−1)>I _(out)(i−2) AND I _(out)(i−1)*0.4>I _(d)(i−1)  (15)the AND gate will issue a signal that will operate a switch, which putsI_(out) equal to I_(in). At the same time a delay unit is triggered,that will hold the switch in that state for 12 ms. The signal from thedelay unit is sent to a second AND gate. A further logical unit isconnected to the second AND gate. This logical unit compares the presentI_(in)(i) value with the set1 value or that the present I_(in)(i) valuedecreases. If this is true and a signal is sent from the delay unit, asignal is issued from the second AND gate to reset the system. Thefunction of the delay unit is to provide a timer to smooth thecompensated outgoing current as seen in FIG. 5.

The results of the current compensation is shown in FIG. 5 where thecurve marked with □ represents the incoming current, the curve markedwith o represents the outgoing current and the curve marked with Δrepresents compensated current. Here, the outgoing current becomes zeroafter 1.5 ms when CT saturates and the compensated current is almostequal to the incoming current during external fault conditions.

If the compensated current I_(comp) is used to calculate the integratedoutgoing current, an almost perfect compensation result can be obtained.The new integrated values are obtained from the following equations.$\begin{matrix}{I_{OUTc} = {\int_{t1}^{({{t1} + T})}{I_{comp}\ {\mathbb{d}t}}}} & (16)\end{matrix}$ I_(Rnew)=I_(OUTc)(17)I _(Dnew) =I _(IN) −I _(OUTc)  (18)

With the new integrated differential current I_(Dnew) and restrainedcurrent I_(Rnew) it is very easy to build a stable differentialprotection algorithm based on percentage restrained criterion becausethe differential current I_(Dnew) is equal to zero in cases of externalfaults and normal outgoing conditions. Finally a tripping criterion canbe built based on these values with equation (19) below where thestability factor k is fixed around 0.5.I _(Dnew) −k×I _(Rnew)>0  (19)

This method and algorithm may also be used to detect simultaneous faultsas long as the internal fault current is higher than the external faultcurrent.

FIG. 6 schematically shows how the method according to the invention maybe implemented in a power system. A busbar 10 is connected to a numberof transmission lines 12, where some are incoming lines connected topower sources and some are outgoing lines connected to loads. Theconnection of the transmission lines to the busbar is considered to bethe protection zone PZ.

Each transmission line is arranged with a current transformer CT. Eachtransmission line is further provided with a breaker 13, capable ofbreaking the connection. The CTs are connected to a fast tripping device14 via lines 16. The CTs are designed to provide currents that areproportional to the currents of the transmission lines. The fasttripping device comprises means for carrying out the steps of measuringthe currents, calculating the differential current, integrating thecurrents, differentiating the integrated values in order to detectexternal faults and simultaneous faults. The tripping signal istransmitted to all breakers arranged on the transmission lines via line18.

The fast tripping device may comprise filters for filtering the signals,converters for sampling the signals and one or more micro computers. Themicro processor (or processors) comprises a central processing unit CPUperforming the steps of the method according to the invention. This isperformed with the aid of a dedicated computer program, which is storedin the program memory. It is to be understood that the computer programmay also be run on a general purpose industrial computer instead of aspecially adapted computer.

The software includes computer program code elements or software codeportions that make the computer perform the method using equations,algorithms, data and calculations previously described. A part of theprogram may be stored in a processor as above, but also in a ROM, RAM,PROM or EPROM chip or similar. The program in part or in whole may alsobe stored on, or in, other suitable computer readable medium such as amagnetic disk, CD-ROM or DVD disk, hard disk, magneto-optical memorystorage means, in volatile memory, in flash memory, as firmware, orstored on a data server.

It is to be understood that the embodiments described above and shown onthe drawings are to be regarded as non-limiting examples of the presentinvention and that it is defined by the appended patent claims.

1. A method for current compensation of a protection system forprotecting a zone in a power system having a number of incomingtransmission lines connected to power sources and a number of outgoingtransmission lines connected to a number of loads where the powersources and the loads are arranged outside the zone and a number ofcurrent transformers (CT) responsively coupled to the transmissionlines, comprises the steps of: continuously measuring all incomingcurrents (I_(in)) to the zone, continuously measuring all outgoingcurrents (I_(out)) from the zone, continuously calculating adifferential current (I_(d)) according toI _(d) =I _(in) −I _(out), continuously calculating q=I_(out)/I_(in),and continuously comparing q with set values, continuously comparingI_(out) with a set value Set1, setting I_(out)=I_(in) at an instant whenq changes and I_(out) exceeds a value Set1, which is an indication ofthe occurrence of a fault external to the protection zone due tosaturation of the CT of the faulted outgoing transmission line, andusing the value I_(out) as a compensated current I_(comp) in theprotection system in order to prevent tripping of the protection systemdue to external faults.
 2. Method according to claim 1, comprisingcontinuously integrating said value I_(comp) according toI_(OUTc) = ∫_(t1)^((t1 + T))I_(comp) 𝕕t, continuously puttingI_(Rnew)=I_(OUTc), continuously calculating I_(Dnew)=I_(OUT)−I_(OUTc),and continuously comparing I_(Dnew) and I_(Rnew) according toI_(Dnew)−k×I_(Rnew)>0.
 3. The method according to claim 2, wherein thefactor k is set in the range 0.3–0.8.
 4. The method according to claim1, wherein the incoming current and outgoing current are sampled andwherein the method comprises the steps: (1) evaluating q according toq(i−2)>0.6q(i−1)>0.6q(i)<0.8 (2) evaluating I_(out) according toI _(out)(i−1)>Set1, (3) evaluating I_(in) according toI _(in)(i)>I _(in)(i−1), and (4) evaluating I_(out) according toI _(out)(i−1)>I _(out)(i−2) wherein I_(comp) is a compensated currentandI _(out)(i−1)×0.4>I _(d)(i−1), wherein (i) is the present samplingoccurrence, and setting I_(out)=I_(in) if all criteria (1)–(4) arefulfilled.
 5. The method according to claim 4, further comprising thesteps of: (5) evaluating I_(in) according toI _(in)(i)<Set1 orI_(in) decreases, and resetting I_(out) to its actual value when (5) isfulfilled.
 6. The method according to claim 1, comprising maintainingI_(out)=I_(in) for a selected time after I_(out) has been set to I_(in).7. The method according to claim 6, wherein the selected time is 12 ms.8. A device for current compensation of a protection system forprotecting a zone in a power system, having a number of incomingtransmission lines connected to power sources and a number of outgoingtransmission lines connected to a number of loads where the powersources and the loads are arranged outside the zone and a number ofcurrent transformers coupled to the transmission lines, comprising meansfor continuously measuring all incoming currents (I_(in)) to the zone,means continuously measuring all outgoing currents (I_(out)) from thezone, means for continuously calculating a differential current (I_(d))according to I_(d)=I_(in)−I_(out), means for continuously calculatingq=I_(out)/I_(in), and means for continuously comparing q with a setvalue S, and means for continuously comparing I_(out) with a set valueset1, means for setting I_(out)=I_(in) at the instant when q changes andI_(out) exceeds the value set1, which is an indication of the occurrenceof a fault external to the protection zone due to saturation of the CTof the faulted outgoing transmission line, and means for using the setvalue of I_(out) as a compensated current I_(comp) in the protectionsystem in order to prevent tripping of the protection system.
 9. Adevice according to claim 8, further including means for continuouslyintegrating said value I_(comp) according toI_(OUTc) = ∫_(t1)^((t1 + T))I_(comp) 𝕕t, means for continuously puttingI_(Rnew)=I_(OUTc), means for continuously calculatingI_(Dnew)=I_(OUT)−I_(OUTc), and means for continuously comparing I_(Dnew)and I_(Rnew) according to I_(Dnew)−k×I_(Rnew)>0.
 10. A computer programproduct comprising computer code means or software code portions formaking a computer or processor perform the steps of: continuouslymeasuring all the incoming currents (I_(in)) to the zone, continuouslymeasuring all the outgoing currents (I_(out)) from the zone,continuously calculating the differential current (I_(d)) according toI _(d) =I _(in) −I _(out), continuously calculating q=I_(out)/I_(in),continuously comparing q with set values, continuously comparing I_(out)with a set value Set1, setting I_(out)=I_(in) at the instant when qchanges and I_(out) exceeds the value S, which is an indication of theoccurrence of a fault external to the protection zone due to saturationof the CT of the faulted outgoing transmission line, and using the setvalue of I_(out) as a compensated current I_(comp) in the protectionsystem in order to prevent tripping of the protection system.
 11. Acomputer program product according to claim 10, further comprisingcontinuously integrating said value I_(comp) according toI_(OUTc) = ∫_(t1)^((t1 + T))I_(comp) 𝕕t, continuously puttingI_(Rnew)=I_(OUTc), continuously calculating I_(Dnew)=I_(OUT)−I_(OUTc),and continuously comparing I_(Dnew) and I_(Rnew) according toI_(Dnew)−k×I_(Rnew)>0.
 12. A computer program product according to claim10, further comprising sampling the incoming current and outgoingcurrent and performing the steps of: (1) evaluating q according toq(i−2)>0.6q(i−1)>0.6q(i)<0.8 (2) evaluating I_(out) according toI _(out)(i−1)>Set1, (3) evaluating I_(in) according toI _(in)(i)>I _(in)(i−1), and (4) evaluating I_(out) according toI _(out)(i−1)>I _(out)(i−2) andI _(out)(i−1)×0.4>I _(d)(i−1), wherein (i) is the present samplingoccurrence, and setting I_(out)=I_(in) if all criteria (1)–(4) arefulfilled.
 13. A computer program product according to claim 12, furthercomprising performing the steps of: (5) evaluating I_(in) according toI _(in)(i)<Set1 orI_(in) decreases, and triggering a timer at the moment when I_(out) isset to I_(in), and resetting I_(out) to its actual value when (5) isfulfilled and the timer has ended.
 14. Use of a computer program productaccording to claim 10, to provide protection measures for a protectionzone of a power system.
 15. A computer readable medium comprisingcomputer code means according to claim
 10. 16. Use of a device accordingto claim 8 to provide protection measures for a protection zone of apower system.